Generally, vehicles are equipped with retarders in order to have the benefit of a form of dynamic supplemental braking means which has great endurance despite being of limited capacity and having an effectiveness which falls at low speeds. The use of such retarders saves foundation (friction) brake lining wear, particularly at low braking demands. The conventional practice is for retarders to be under separate control from the foundation brakes, usually from a driver-operated hand control.
It is known from GB-A-2 080 458 to provide an electronic braking system in which control of the foundation brakes and transmission shaft or engine retarder is coordinated in response to the braking demand signal. In the system of GB-A-2 080 458, the retarder receives the full brake demand signal and is operated in an open loop, whereas the foundation friction brakes are operated in a closed loop and receive the deceleration error signal. A problem in practice with such a system, wherein the retarder receives full braking demand and a closed loop acceleration system is employed, is that, because of system delays and inherent lack of responsiveness in air brake actuation, system instability can be caused, particularly if wear, corrosion and poor maintenance cause hysteresis levels to increase. A further problem with the system of GB-A-2 080 458 arises from closing the loop using deceleration signals which are inherently noisy, being derived by a differentiation process on signals which are subject to substantial load noise disturbances. This is because the filtering required to remove much of the noise generated, results in a slow deceleration signal not adequate for dealing with fast changes in braking demand which is required of road vehicles under critical circumstances.